Single Core 230Volt Supply??

[Piratepete]

A farmer client has a supply cable going underground from the DNO's kennel to a large barn converted to stables with power and light. The cable is fed from a 30 Amp switch fuse and a preceeding 30 mA RCD.

In the barn, I noted that though the power and light were on separate 2.5mm and 1.5 mm circuits these were connected together to the incoming cable by a large choc block wrapped in insulating tape! There's a separate 6mm earth wire also into said block (TT supply).

So on replacing this little mess with a 2 way garage unit I discover that the supply cable is not a 2 core SWA that it looked like, but is single core aluminium with a stranded copper insulated outer sheaf which is being used as the neutral conductor.

Does anyone recognise this type of cable and know it's normal use please?
How do I work out it's current rating?
Can I leave it? Ever tried getting a farmer to spend money on something that's been working fine for years!

Further info. It's stamped Crompton 600volt. IR to earth is 930Meg for inner core but only 3Meg for the outer neutral (NO surprise!).

Pete

 

[Leesparkykent]

sounds like concentric cable.

 

[Simonslimline]

The DNO's use it a lot.

Straight & split concentric cable.

 

[Piratepete]

Hi Lee
Yes, it is concentric but never come across it before in my sheltered 'Domestic' life. The core is at least 16 sq mm, so no doubt good for 30 Amps. But it's installed under a concrete yard. Suppose it's ok if it's mecanically protected in a duct but all I can see is it disapearing into solid earth!

 

[Leesparkykent]

Hi Lee
Yes, it is concentric but never come across it before in my sheltered 'Domestic' life. The core is at least 16 sq mm, so no doubt good for 30 Amps. But it's installed under a concrete yard. Suppose it's ok if it's mecanically protected in a duct but all I can see is it disapearing into solid earth!

Yeah not ideal from the regs POV but the DNO bury it all the time.

 

[Dave OCD]

Yeah there will be literally thousands of miles of the stuff buried all over the country - should be in ducting but often isn't continuously to say the least ! I've seen it buried direct around 75mm below a road surface before ! :yesnod:

 

[davesparks]

Hi Lee
Yes, it is concentric but never come across it before in my sheltered 'Domestic' life. The core is at least 16 sq mm, so no doubt good for 30 Amps. But it's installed under a concrete yard. Suppose it's ok if it's mecanically protected in a duct but all I can see is it disapearing into solid earth!

No it is not suitable for burial, or even any use, within a consumers installation.
It does not have an earthed metallic armour, nor does it have any earth conductor. This cable is only suitable for use in a TNC system, which it is illegal to install in a consumers installation under the esqcr

 

[Dave OCD]

No it is not suitable for burial, or even any use within a consumers installation.
It does not have an earthed metallic armour, nor does it have any earth conductor. This cable is only suitable for use in a TNC system, which it is illegal to install in a consumers installation under the esqcr

Try telling that to all the DNOs in the country Dave.

 

[davesparks]

Try telling that to all the DNOs in the country Dave.

The DNOs already know that it is not suitable for burial within a consumers installation, why would they need telling.

 

[Dave OCD]

The DNOs already know that it is not suitable for burial within a consumers installation, why would they need telling.

No you know what I mean, they bury it everywhere on the distribution network.

 

[telectrix]

Then moan when bob the builder puts his jcb bucket through it

 

[7029 dave]

Then moan when bob the builder puts his jcb bucket through it

And charge megger bucks.

 

[davesparks]

No you know what I mean, they bury it everywhere on the distribution network.

I don't know what you mean, no.

Concentric cable such as this is not suitable for burial in a consumers installation but is suitable for burial in a DNOs installation. The difference is quite obvious is it not?

For direct burial of a cable an earthed metallic screen or armour is required, in a concentric cable such as this that can only be achieved by connecting the outer conductor as a protectively earthed neutral (PEN or combined neutral and Earth CNE) this would be a TNC system.
Within a consumers installation it is not permitted to use a TNC system therefore this cable cannot be used. Within a DNOs installation a TNC system is permitted and is widely used so this cable is suitable.

 

[Dave OCD]

Most Utilities have fairly good network plans showing location of cables/pipes etc but they're never 100% accurate.

Edit - in reply to 7029 Dave's post.

 

[Dave OCD]

I don't know what you mean, no.

Concentric cable such as this is not suitable for burial in a consumers installation but is suitable for burial in a DNOs installation. The difference is quite obvious is it not?

For direct burial of a cable an earthed metallic screen or armour is required, in a concentric cable such as this that can only be achieved by connecting the outer conductor as a protectively earthed neutral (PEN or combined neutral and Earth CNE) this would be a TNC system.
Within a consumers installation it is not permitted to use a TNC system therefore this cable cannot be used. Within a DNOs installation a TNC system is permitted and is widely used so this cable is suitable.

My whole point was you said it was not suitable for burial Dave, simple as that. And in the case of a TNS supply the cable would be split concentric so only a part of the outer cable circumference would be earthed below the outer sheath.